Continuous casting is the process whereby liquid steel is solidified into a “semifinished” billet, bloom, beam blank or slab for subsequent processing in a steel hot rolling mill. A billet has a square or round cross section with a typical area of about 23,226 mm2; a slab has an even larger rectangular cross section; a beam blank is a near-net shape product used to feed medium and heavy section mills; and a bloom has a rectangular or round cross section with a cross sectional area larger than a billet, but smaller than that of a slab. FIG. 1 is a schematic diagram of an example slab continuous casting process 100. Once the steel has been refined in ladle 102 to achieve the desired chemical composition and temperature, ladle 102 is transported to the caster for casting. During casting, the steel flows from the bottom of ladle 102 through ladle shroud 103 into a holding bath called a tundish 104. Tundish 104 allows a reservoir of steel to continuously feed mold 108 as one ladle 102 is emptied and a new one is opened. With this arrangement, several ladles 102 of liquid steel of the same grade or closely-related grades can be continuously cast before the caster is turned around to continuously cast another sequence of heats. One ladle of steel is referred to as one heat and several ladles of steel of the same grade or closely-related grades cast continuously in this manner is referred to as a sequence of heats.
The initial solidification of a slab cast in this manner begins in mold 108, a rectangular box that may be made of copper or copper-based alloy. Water jackets may be mounted on the four sides of the mold to facilitate solidification. Mold 108 may be only about 800-900 mm long, and at its bottom, the thickness of the solidified steel 118 (referred to as the shell) may be a few millimeters thick depending on the nature of cooling in the mold. The partially solidified product 116 (referred to as the strand) is essentially like a water tank with outer solid shell and liquid interior. Strand 116 is continuously withdrawn into the secondary cooling chamber of the caster, which comprises sets of supporting water-cooled rolls 114 arranged in segments. Water spray nozzles 112 are arranged in between the rolls 114 to gradually continue and complete the solidification of the strand 116. Once solidification is complete, strand 116 is cut to length (e.g., by a torch) at cutoff point 120 and may be one of a billet, a bloom, a beam blank or a slab 200. By the time strand 116 is completely solidified, it may have traveled several tens of meters from the location of mold 108. Hence, the caster is typically designed as a curved machine to allow for space and to facilitate a better roll support system. The curved strand 116 is straightened in the horizontal portion of the machine before it is cut to length. The steel undergoes different thermal states and phase changes and experiences different degrees of mechanical stresses before casting is complete. As a result of all of these processes, the semi-finished product may exhibit certain surface and internal defects due to thermal and mechanical stresses. A defect is an imperfection or a flaw in as-cast products that could deteriorate the performance of the products and render them unsuitable for their intended applications.
FIG. 2 shows examples of various types of defects commonly found in an as-cast steel product 200. Such defects include midway 201, triple-point 202, centerline 203, diagonal 204, straightening/bending 205, pinch roll surface cracks 206, mid-face longitudinal 207, corner longitudinal 208, mid-face transverse 209, corner transverse 210 and star 211 defects. The severity of these defects varies depending on casting-specific conditions. By visually inspecting the as-cast product, potential locations in the machine where these defects originate may be examined and adjusted before subsequent casting sequences. However, such diagnoses may be inadequate, due in significant part to the industry's lack of a quantitative method of evaluating the defects.
Historically, the rating of the severity of internal slab defects has been conducted by a manual, subjective and visual comparison of etched cross sections of as-cast products to the Mannesmann charts, which were developed in the 1970s. FIG. 3 shows examples of the Mannesmann charts for various centerline segregations, varying sequentially by degree. As shown, the Mannesmann charts provide a scale of five ratings denoted by the integers 1 to 5, corresponding to increasing degrees of segregation. Similar charts also exist for other types of defects, such as longitudinal (radial) internal cracks, transverse (halfway) internal cracks, narrow side internal cracks, corner internal cracks, cloud-shaped inclusions and spot-shaped inclusions. (See FIGS. 10 to 15.) Thus, when an operator is evaluating a product for the extent of, for example, centerline segregation, the operator will visually inspect the product and assign it a value between 1 and 5 based on the operator's subjective opinion of which image on the Mannesmann chart the product most closely resembles. Sometimes if the extent of segregation falls squarely between two of the Mannesmann images, the operator will assign a half-integer value, such as 2.5, to the product.
Studies show that this technique is flawed due to the inconsistent, subjective interpretations of the charts by different operators. For example, in 2008, the Pipeline and Hazardous Material Safety Administration (PHMSA) concluded that the application of Mannesmann charts for rating centerline segregation is highly subjective. By way of example, FIGS. 4A and 4B show the results of two separate sets of round-robin testing conducted by two operators. As shown, the ratings selected by the two operators varied greatly—at times by as much as a full point (i.e., a 25% difference) on the Mannesmann scale. Specifically, the correlation coefficient for the first set of round-robin testing in FIG. 4A was only 0.32, and the correlation coefficient for the second round of testing in FIG. 4B was only 0.47. Hence, the lack of reproducible results from the visual and highly subjective assessment of internal defects makes process and product developments ineffective. Further, because operators typically rate products on either a whole number or, at most, a half-whole number basis, the granularity of the Mannesmann rating scale is relatively low.